In the following description and in the appended claims, by the term ‘coolant’ a medium is designated that, when used in a cooling system, transports cold or heat substantially without change of phase between sites of differing temperature (though short-term local changes of phase in the coolant medium may occur). This holds, of course, for the operating conditions for which the system in question is designed, since a change of phase can always be brought about under sufficiently extreme conditions. Coolants are generally present in the liquid state. Water mixed with alcohol or with another antifreeze agent may, for example, be employed as coolant.